Adjustable charge bars

[lotech]

Is there a general opinion on the charge -to-charge consistency of adjustable charge bars? I've used one for a number of years on a Universal and it throws fairly consistent Bullseye charges. I did some checking recently with another adjustable bar (had it a while, too) on a Progressive machine trying both Bullseye and 231. Charges may vary up to half a grain, though most variances aren't that large. May get ten or more acceptable charges followed by one or two way too heavy or too light. 

I'm familiar with consistent technique, etc., but am at a loss to explain the variances. Nothing appears to be loose. Any ideas, or are is this to be expected with adjustable bars?

[NYKenn]

I have used adjustable charge bars for determining a load I liked. Once found, I use a fixed charge bar.

I have used the adjustable, (1 Hoag, 1 Barreltester) to load up to a box at a time, and the loads have been just as consistent as a fixed bar. There has been some variance, but minimal at best that I attribute to the powder.

When I first used the adjustable, I weighed the charge every ten rounds. Now I only weigh the first 3 in the beginning and the last 3 at the end to insure consistency.